In our Thang Component System architecture, the Systems fulfill three roles.

1. Systems are unto Levels as Components are unto Thangs. For example, if I want my Level to have gravity, I add the Movement System and configure its gravity to 9.81 m/s^2.
2. They also serve as namespaces for the Components. As an example, the Physics System doesn't actually do anything on its own yet, but it was a good place to put physics.Physical Component, which controls the size, shape, rotation, and position of all physical Thangs.
3. They can coordinate behavior across those Components. For example, the Collision System will look at all the Thangs which have the collides property set and update its Box2D-based collision simulation once per frame, instead of each Thang with a Collides Component trying to figure out its own collision interactions.

You can also live-edit the code for any System directly within the Systems tab of the Level editor, just like in the Component example.

To see all the entity data for, say, the Movement System, I can just go to http://localhost:3000/db/level_system/movement.